Technical Field
The present invention relates to a near-field probe for a broadband spectral measurement with a nanometer-level resolution, and more particularly, to a tuning-fork based near-field probe which enables a spectral measurement by preventing multiple reflections by the antenna effect of a probe unit from appearing and by reducing the amount of generation of background scattering signals at portions, except for the tip of a nano-probe; a near-field microscope using the near-field probe; and a spectral analysis method using the near-field microscope.
Background Art
An optical microscope is used to view the structures of a living body or a nano-device in units of nanometers, or the shape of the surface thereof. Such an optical microscope allows an object to be viewed with light, so that there is a limit in resolving power due to the limit of diffraction. Generally, the resolving power of an optical microscope is limited to about a half of the wavelength of light used in the microscope.
With the appearance of a near-field microscope (NFM), the limit of diffraction is overcome, so that it becomes possible to obtain an optical image with a resolution less than the wavelength of light. A near-field microscope is a device which is configured: to have a structure in which force microscope technology using a probe, as an atomic force microscope (AFM), and optical measurement technology are combined; and to read, through the tip of a probe, a near field existing within a distance less than the wavelength of light from the surface of a material to be inspected. Differently from an electric field or a magnetic field propagating in a free space, a near field, which is restrained on the boundary surface between materials, can exist in the distribution of a size less than the wavelength. Since such a near field does not propagate into a free space, it is impossible to view from an exterior, but it is possible to read restrained light through a nano-sized probe.
There are various types of near-field microscopes, wherein a near-field microscope can be implemented by utilizing the probe of an atomic force microscope (AFM) as a probe for a near field. An AFM enables acquisition of the topography of a sample with a nano-level high resolution; combining an optical method therewith enables the distribution of an optical constant (e.g. a permittivity, a magnetic permeability, or the like) of a material to obtained with a high resolving power which is almost the same level as the resolution of the AFM; and, through this, what is the material can be identified.
At present, the highest resolving power is achieved by an apertureless type near-field microscope. This technology is implemented in such a manner as to incident light on the nano-probe unit of an AFM or a force microscope similar to the AFM, and to remotely measure a scattered wave generated from the tip of the probe unit. Depending on the wavelength of used light, a wavelength at which an optical constant is to be measured is determined. When a broadband variable-wavelength light source is used, an optical constant spectrum can be obtained in a wide wavelength band, thereby implementing broadband spectroscopy with a nanometer-level resolution.
Meanwhile, a terahertz frequency band corresponds to electromagnetic waves having the frequencies of 0.1 to tens of THz, and is located between an optical wave and an electronic wave. At present, research and development for light sources, detectors, and application technology in connection with the frequency are being actively conducted.
As spectral technology in the terahertz frequency band, a terahertz time-domain spectroscopy (THz-TDS) using a pulse light source is widely used. A pulse light source may be regarded as a broadband light source the phase of which accords in view of a frequency band. When a broadband pulse in a terahertz band passes through a specific material, the spectral characteristic of the material is reflected in the pulse, thereby causing a change in the pulse. By analyzing the change, a broadband terahertz spectral measurement on a sample is completed.
FIG. 1 is a block diagram illustrating a conventional terahertz time-domain spectroscope to which the terahertz time-domain spectroscopy is applied. As shown in FIG. 1, the conventional terahertz time-domain spectroscope includes an ultrafast pulse laser 10, a plurality of beam splitters 11, a mechanical chopper 12, a mechanical optical delay line 13, an InAs wafer 14, a sample 15, a terahertz detector (THz detector) 16, a lock-in amplifier 17, and a control personal computer (control PC) 18.
Referring to FIG. 1, the fast-pulse laser 10 generates a terahertz pulse. The terahertz pulse is generated by the InAs wafer (THz emitter) 14 through the plurality of beam splitters 11, the mechanical chopper 12, and the mechanical optical delay line 13. Accordingly, a terahertz pulse having a bandwidth of about 0-2.5 THz with duration of 0-1 ps is generated from the InAs wafer 14. In order to generate a terahertz wave, an electro-optic crystal (EO-crystal), a photoconductive antenna (PC antenna), or the like may be used, instead of the InAs wafer 14.
The terahertz pulse generated as above penetrates the sample 15 on a moving stage through a parabolic mirror or the like. Then, the terahertz pulse which has penetrated the sample 15 is inputted to an electrode unit of the terahertz detector 16. At the moment when the terahertz pulse is inputted to the terahertz detector 16 through the route, the terahertz detector 16 comes to have conductivity, wherein the duration of the conductivity is much shorter than the duration of the terahertz pulse.
In the terahertz detector 16, current flows by an electric field generated by the input terahertz pulse (i.e. probe pulse), in which the amount of current is proportional to the intensity of the electric field. For the terahertz detection, an electro-optic crystal (EO crystal) may be used instead of the photo-conductive antenna.
Since the time interval between the terahertz pulse generated by the InAs wafer through the photoexcitation by the femtosecond laser 10 and the probe pulse inputted to the terahertz detector 16 can be adjusted through the mechanical optical delay line 13, the terahertz detector 16 can measure the intensity of the electric field in a time domain.
The intensity of the electric field measured by the terahertz detector 16 is inputted to the control PC 18 through the lock-in amplifier 17, and a measured time-domain signal is Fourier-transformed by the control PC 18 and is analyzed in a frequency domain. Accordingly, the broadband sample characteristic of the sample 15 can be measured.
An optical image of the sample 15 can be obtained by raster-scanning the sample through the moving stage. In this case, the resolution is determined by the size of the focus of a terahertz wave focused on the sample 15, and is limited to a half of wavelength.
In order to overcome such limitation in resolution, various attempts have been made. Representatively, a concept of a terahertz scattering-type scanning near field optical microscope (THz s-SNOM) capable of measuring a near filed in a terahertz band by combining a terahertz time-domain spectral system using a broadband pulse light source with a non-aperture type near-field microscope system, as shown in FIG. 1, has been proposed and researched.
In the THz s-SNOM, a sample is attached on a piezo-stage, and a nano-probe is mounted above the sample and vibrates in a perpendicular direction with respect to the sample. A cantilever-type near-field probe widely used in a commercial AFM has been generally used as the nano-probe.
FIG. 2 illustrates conventional technology of using a cantilever-type near-field probe, wherein, as illustrated therein, a structure in which a probe 22 is connected to a one-side end of a cantilever 21 in a direction perpendicular to the cantilever 21 is provided.
A PID control is performed on the piezo-stage, and thus the average distance between the sample and the tip 22-1 of the probe is constantly maintained. In such a state, the topography of the sample can be obtained by scanning the sample.
The THz detector measures the mixed signal of a wave reflected from the surface of the sample, and a scattered wave from the nano-probe. Generally, since the scattered wave at the tip 22-1 of the nano-probe 20 is extremely weak as compared with an incident wave, it may be difficult to extract a scattered wave at the tip 22-1 of the nano-probe 20 from a measured signal. The non-linear characteristic of the scattered wave is used to measure the scattered wave.
FIG. 3 illustrates the non-linear characteristic of the scattered wave. As shown in FIG. 3, the scattered wave at the tip 22-1 of the nano-probe 20 has a characteristic that the scattered wave non-linearly increases as the distance “g” between the tip 22-1 of the probe and the sample 23 is decreased. This results from a near-field interaction between the tip 22-1 of the probe and the sample 23.
Therefore, as illustrated in FIG. 4, the scattered wave at the tip 22-1 of the probe can be detected by obtaining a difference between a waveform detected by the terahertz detector when the tip 22-1 of the probe is close to the sample 23, and a waveform detected by the terahertz detector when the tip 22-1 of the probe is relatively far away from the sample 23. The scattered wave shows a characteristic that is extremely susceptible to the distance between the tip 22-1 of the probe and the sample 23. In contrast, other components of terahertz wave which is detected by the detector, such as reflected waves and the like, are not largely influenced by the distance between the tip 22-1 of the probe and the sample 23. Therefore, with a difference as described with respect to FIG. 4, a scattered wave component from the tip 22-1 of the probe can be extracted. In practice, a scattered wave modulated by a vibrating nano-probe is demodulated using a lock-in amplifier, so that a measurement is achieved.
FIG. 5 illustrates various scattering sources which exist in a conventional THz s-SNOM system. When the vibration frequency of a probe is set as “Ω”, a signal modulated by the probe may be expressed as Equation 1 below.
                              E          ⁡                      (            t            )                          =                                            ∑              n                        ⁢                                                  ⁢                                          E                b                n                            ⁢              cos              ⁢                                                          ⁢              Ω              ⁢                                                          ⁢              t                                +                                    ∑              n                        ⁢                                                  ⁢                                          E                Tip                n                            ⁢              cos              ⁢                                                          ⁢              Ω              ⁢                                                          ⁢              t                                                          (        1        )            
As expressed in FIG. 5 and by Equation 1, a scattered wave Eb generated from the body or base part of a cantilever-type nano-probe 20 is modulated by an angular velocity “Ω”, together with a scattered wave generated from the tip 22-1 of the nano-probe 20. Thus, when a signal is demodulated through the lock-in amplifier, the scattered wave Eb is reflected in a demodulated signal generated through the lock-in amplifier although the scattered wave Eb is not the scattered wave generated from the tip 22-1 of the nano-probe 20. This is called “background scattering”. In order to remove such background scattering and to extract a scattering signal, a harmonic of “2Ω” or “3Ω” has been used instead of “Ω”.
In a THz s-SNOM system including a broadband pulse light source and an electric field detector, when a terahertz time-domain scattered wave is measured, the body part 22 of a probe functions as a wave guide. Accordingly, a phenomenon that an incident wave is propagated to the base part 22-2 of the probe along the body 22 of the probe appears.
Then, the wave arriving at the base part is reflected to generate second and third scattering signals. The scattering signals generated as above complicate the analysis of a frequency domain, thereby disabling a spectral analysis through a Fourier transform of a measured signal.
FIG. 6 is a view illustrating a wave guide effect and a multiple reflection phenomenon of the base part of a probe in a conventional cantilever-type nano-probe.
FIG. 8 is a view illustrating a scattered wave generation area in a conventional cantilever-type nano-probe. Referring to FIG. 8, in a long wavelength range such as a terahertz band, the beam radius of a focus of a terahertz beam focused on a nano-probe on a sample by a parabolic mirror is larger than the structure of a cantilever-type nano-probe due to the limit of diffraction. For this reason, it is inevitable to generate background scattering, and thus many difficulties are caused in measuring the optical characteristic of an object to be measured.
In addition, since scattering signals are periodically generated due to the multiple reflections and the wave guide effect of the cantilever-type nano-probe, the broadband information of a sample is mixed with the periodically scattering signals, and thus there is a difficulty in a quantitative broadband analysis. Referring to “(a)” of FIG. 7, multiple pulses appear close to each other in a time domain due to multiple reflections caused on the base part of a nano-probe, as shown in FIG. 6, and a transient response of a first pulse is mixed with second and third pulses, thereby making it impossible to separate the transient response by the first pulse. In addition, as shown in “(b)” of FIG. 7, although a Fourier transform is performed for a frequency analysis, it is impossible to analyze the broadband characteristic of the sample due to the antenna effect of the nano-probe.